Implement server-side support for inlay hints using CollectVariableType

[fwcd]

This is an early draft of support for inlay hints that uses the new CollectVariableType SourceKit request.

Implementation Progress

• Add UIDs and structures for CollectVariableType
• Support ranged CollectVariableType requests
• Reimplement inlay hints using CollectVariableType
• Update existing tests
• Add new test for inlay hints on closure parameters

Note that this also requires a very recent build of the Swift toolchain, otherwise the tests will fail

[fwcd]

I have tested this branch with the newest toolchain and it's looking pretty nice already:

There are still two minor issues that probably have to be addressed on the SourceKit-side:

• The flag that determines whether a variable declaration has an explicit type doesn't seem to work if the declaration is inside an if/guard/while-let, interestingly it seems to work correctly with if/guard/while-case-lets however
  • This is addressed in [SourceKit] Determine correctly whether typed variables in if/guard/while-let-statements have explicit type annotations swift#38084
• Wildcard patterns (_) seem to generate a position that is off by one character
  • The issue seems to be that wildcard patterns cause the DeclNameRange to have length 0
  • This is addressed in [IDE] Fix name range of wildcard declarations swift#38085

[ahoppen]

LGTM

[ahoppen]

@swift-ci Please test